At present, there are chiefly two processes for producing phosphoric acid in industry. (1) producing phosphoric acid with a wet process: using sulfuric acid to decompose phosphate ore to obtain dilute phosphoric acid and solid slag (briefly called phosphogypsum) with CaSO4.nH2O as a main component, and concentrating the dilute phosphoric acid to obtain wet-process phosphoric acid with about 54% phosphoric acid. This process has the following major drawbacks: the first drawback is large consumption of sulfuric acid; the second drawback is that the slag phosphogypsum cannot be used effectively, and sulfuric acid, phosphoric acid and soluble fluorides entrained therein are all soluble in water, and rain wash of the slag piled in the nature is apt to cause serious pollution to the environment; the third drawback is that the resultant phosphoric acid contains higher contain of impurities and is generally only used to produce fertilizer; and the fourth drawback is that high-grade phosphate ore must be used to ensure economy of the product. (2) producing phosphoric acid with a hot process: first, placing phosphate ore, silica and carbonaceous solid reductant in an ore-smelting electric furnace, raising a temperature in the furnace to 1300° C. with energy of electric arc formed by electrical short-circuiting, reducing phosphor in the phosphate ore in the form of P4, meanwhile converting carbonaceous solid reductant into CO, washing gas mainly containing P4 and CO discharged out of the ore-smelting electric furnace with water, cooling P4 into solid to separate from gas phase to obtain the product yellow phosphorus, igniting and burning exhaust gas containing CO at an outlet of a chimney and exhausting to the atmosphere; heating the obtained P4 to 80° C. to change it into liquid phase, subjecting it to oxidization combustion reaction with introduced air in a hydration tower to obtain phosphoric anhydride P2O5, and then absorbing it with water to obtain phosphoric acid. The hot-process production of phosphoric acid has the following main drawbacks: the first drawback is large consumption of electrical energy; the second drawback is that gas discharged out of the ore-smelting electric furnace, from which P4 is already separated, still entrains a large amount of fluorides (existing in the form of SiF4 and HF) and a small amount of un-deposited gas P4, which causes serious pollution to the atmospheric environment; the third drawback is that gas containing a large amount of CO is directly burnt and exhausted, which causes large waste of energy; the fourth drawback is that high-grade phosphate ore needs to be used to ensure economy of the production.
To overcome impact on production of phosphoric acid exerted by shortage of electrical energy, insufficient pyrites resources and gradual reduction of high-grade phosphate core, Occidental Research Corporation of the United States proposed a KPA process in 1980's, i.e., a process of producing phosphoric acid with a rotary kiln (briefly called a phosphoric acid producing process with a kiln) (see Frederic Ledar and Won C. Park, et al., New Process for Technical-Grade Phosphoric Acid, Ind. Eng. Chem. Process Des. Dev 1985, 24, 688-697), and carried out a pilot experiment of a pilot device in a 0.84 m (inner)×9.14 m (see the US patent document U.S. Pat. No. 4,389,384). According to this process, phosphate ore, silica and carbonaceous reductant (coke powder or coal powder) are co-ground so that 50%-85% of the co-ground materials passes a −325 mesh, with 1% bentonite being added to produce balls, which are dried and preheated by a chain-type dryer and then delivered into a rotary kiln with a kiln head in which natural gas is burnt, pellets are reduced in the kiln, a maximum solid temperature is controlled as 1400° C.-1500° C., pellet CaO/SiO2 mole ratio is adjusted as 0.26-0.55 so that the a melting point of the pellet is higher than a carbon thermal reduction temperature of the phosphate core in the pellets, phosphor is reduced and volatiles out of the pellets in the form of phosphoric vapor, and then oxidized by air introduced in a middle space of the kiln into phosphorous pentoxide, heat resulting from oxidization is then supplied to the reduction reaction, and finally kiln gas containing phosphorous pentoxide is hydrated and absorbed to obtain phosphoric acid. The idea of the above processing phosphoric acid with a kiln exhibits an excellent industrial application prospect because its principle is to form P4 gas using carbon thermal reduction of the phosphate ore, transfer phosphorus in the phosphate ore to gas phase of the rotary kiln, use a gas-solid separation principle to enable phosphorus to be well separated from other solid substances in the balls, allow the P4 gas transferred into the gas phase of the rotary kiln to go through an oxidization heat liberation reaction with oxygen in the gas phase of the rotary kiln to generate P2O5, supply the liberated heat to carbon thermal reduction (endothermic reaction) of the phosphate ore in the material balls, and finally hydrate and absorb the kiln gas containing P2O5 exiting the rotary kiln to obtain industrial phosphoric acid having a cleanliness much higher than the phosphoric acid produced with the wet process. Since the rotary kiln uses primary energy to maintain phosphate ore carbon terminal reduction temperature, and meanwhile flammable substance P4 generated from the phosphate ore carbon terminal reduction and CO are subjected to the combustion heat liberation reaction in the interior of the rotary kiln to replenish the energy needed to maintain the phosphate ore carbon terminal reduction temperature in the rotary kiln, this process substantially reduces energy consumption as compared with the conventional hot process of producing phosphoric acid.
However, the research indicates that it is very difficult to implement the process of producing phosphoric acid with the kiln in large-scale industrial application and practice and its main drawbacks are as follows:
1. A rotary kiln is an apparatus with a kiln body rotating at a certain speed (0.5 r/min-3 r/min), and it is advantageous in continuously performing mechanical turn and mixture of a solid material fed into the kiln to ensure uniformity of heat reception of the solid material at all locations in the kiln. However, the solid material in the kiln must bear a mechanical frictional force resulting from movement of the material. If a strength of the material is smaller than the received mechanical frictional force, the material can be easily destroyed. A basic principle of the KPA process proposed by ORC corporation of the United States is to co-grind the phosphate ore, the silica and the carbonaceous reductant (coke powder or coal powder) so that 50%-80% of the co-ground materials passes 325 mesh, and then produce them into pellets, the three kinds of substances must be closely copolymerized into a whole so that the mixture does not melt at the carbon thermal reduction temperature of the phosphate ore under the condition the CaO/SiO2 mole ratio in the mixture is 0.26-0.55, and meanwhile carbon reduction of the phosphate ore can be performed smoothly. However, since the reductant carbon is added to the material pellets used in the process, carbon goes through quick oxidization reaction with oxygen in air at a temperature greater than 350° C. to produce CO2. If a conventional method of consolidating pellets at a high temperature (≥900° C.) at a chain grate in the metallurgical industry is employed, the reducing carbon in the pellets will be all oxidized, the pellets entering the rotary kiln will lose the reductant, carbon thermal reduction reaction of the phosphorus naturally cannot be performed, and the process fails as a result. If only the bentonite is added as a bonding agent of the pellets to perform drying and dehydration at a temperature less than 300° C., an anti-pressure strength of the pellets is only about 10 KN per ball, with a falling strength≤1 time per meter; since an acting mechanism of the bentonite is mainly to use interlayer water in its substance structure to adjust a moisture content release speed during the drying of the pellets and improve a burst temperature of the pellets during the drying, and bentonite itself does not play a remarkable role in improving the strength of the pellets. After such pellets are fed into the rotary kiln and before the rotary kiln temperature value reaches 900° C., since the pellets entering the kiln cannot bear the mechanical frictional force resulting from movement of material balls in the pellets, a lot of said pellets are pulverized, and thereafter the phosphate ore powder, silica powder and carbonaceous reductant forming the pellets will separate, the phosphate ore powder after pulverization causes failure of reduction of phosphorus as it cannot get in close contact with carbonaceous reductant. More seriously, once the phosphate ore powder separates from silica powder, its melting point abruptly falls below 1250° C. When such powder-like phosphate ore passes through a high-temperature reducing area (with a material layer temperature of 1300° C. or so) of the rotary kiln, it will totally changes from solid phase into a liquid phase, and thereby adheres to a liner of the rotary kiln to form high-temperature ringing of the rotary kiln, which hinders normal rotation of the materials in the rotary kiln so that a majority of materials added into the rotary kiln overflows from the rotary kiln from a feeding end of the rotary kiln, high-temperature reduction of phosphorus cannot be achieved and the process fails. It can be seen that the raw materials entering the kiln have their intrinsic drawbacks, any industrialized, large-scale or commercialized application of the above-mentioned KPA technology has not yet been witnessed so far.2. Regarding the KPA process with the phosphate ore pellets with carbon being added, a solid material area below a material layer in the rotary kiln belongs to a reduction zone, and a gas flow area of the rotary kiln is above the material layer and belongs to an oxidization zone, the feed pellets are added from a kiln tail of the rotary kiln and discharged out of a kiln head of the rotary kiln by virtue of its own gravity and a frictional force resulting from rotation of the rotary kiln, a burner for burning fuel in the rotary kiln is mounted at the kiln head of the rotary kiln, fume resulting from the burning is introduced out by a blower at the kiln tail, a micro negative pressure is maintained in the rotary kiln, and the gas flow is opposite to a movement direction of the materials. Since there is not a mechanical isolation area between the reduction zone (solid material layer area) and the oxidization zone (the gas flow area above the solid material layer area of the rotary kiln) of the rotary kiln, the material balls exposed on the surface of the solid material layer area and O2, CO2 in the gas flow in the oxidization zone are subjected to convective mass transfer; on the one hand, this causes the reductant in the material balls to be partially oxidized before the material balls are heated by the gas flow heat transfer to the carbon reduction temperature of the phosphate ore so that the material balls are not sufficiently reduced due to shortage of carbonaceous reductant in the reduction zone of the rotary kiln; more seriously, the material balls exposed to the surface of the material layer at the high-temperature area of the rotary kiln is further subjected to chemical reaction with P2O5 already generated from reduction in the kiln gas to produce calcium metaphosphate, calcium phosphate and other metaphosphates or phosphates, thereby causing the phosphorus already reduced into the gas phase to return to the material balls again and form a layer of white crust rich in P2O5 on the surface of the material balls, the layer of crust generally having a thickness of 300 μm-1000 μm, the content of P2O5 in the layer of crust topping 30%; as a result, P2O5 transferred from the material balls to the gas phase does not exceed 60%, which cause a lower yield ratio of P2O5 in the phosphate ore and thereby causes waste of mineral resources and large rise of the phosphoric acid production cost so that the above KPA process losses value in respect of commercial application and industrial spread. Researchers desire gas volatized from the material layer to isolate the reduction zone from the oxidization zone in the rotary kiln, but industrial experiments performed in a rotary kiln with an inner diameter 2 m show that the phenomena of white crust rich in P2O5 on the surface of the pellets still cannot be avoided.
Due to the above-mentioned technical drawbacks, it is still very difficult to use the KPA process proposed by ORC Corporation in large-scale industrial application and practice to produce phosphoric acid.
Joseph A. Megy proposes some improved technical methods with respect to the KPA process (see US patent document U.S. Pat. No. 7,910,080B), i.e., on the premise of maintaining the basis process of KPA unchanged, providing a material stopping ring on a kiln head material discharging end of the cylinder of the rotary kiln to improve a solid material filling rate of the rotary kiln, and meanwhile increasing the diameter of the rotary kiln to reduce a surface area to volume ratio of an inner material layer of the rotary kiln, reduce probability of the material of the material layer being exposed to the surface of the solid material layer to shorten the time that the reductant carbon in the material balls is oxidized by O2 in the kiln gas in the rotary kiln, reduce burn of the reductant carbon before the material balls reach the reduction zone of the rotary kiln and meanwhile decease generation of phosphates or metaphosphates on the surface of the material balls in the high-temperature area of the rotary kiln. In addition, according to the process, it is desired that partial petrol coke is added to the materials entering the rotary kiln so that reducing gas generated by a volatile matter in the petrol coke due to heat reception and volatilization is used to cover between the material layer and the gas flow oxidization area of the rotary kiln to further block the probability of the O2 and P2O5 in the gas flow in the rotary kiln reacting with the material balls to ensure normal operation of the process. However, increase of the filling rate of the rotary kiln allows the material balls to bear larger mechanical frictional force in the rotary kiln, thereby causing a larger proportion of pulverization of the material balls in the rotary kiln, and forming more substances with a melting point lower than the phosphate ore carbon thermal reduction temperature so that the high-temperature ringing of the rotary kiln becomes quicker and more serious and earlier failure of the process is caused. In addition, the volatile matter generated by added small amount of petrol coke is not sufficient to produce sufficient gas and it is difficult to form an effective isolation layer between the solid material layer of the rotary kiln and the gas flow area in the rotary kiln. If an excessive amount is added, the materials in the rotary kiln will entrain a large amount of fuel so that in a slag ball cooling machine in the subsequent process, the redundant fuel is confronted with the air for cooling the slag balls and burns rapidly, a large amount of heat resulting from the burning not only increases the difficulty in cooling the high-temperature slag balls exiting the rotary kiln but also substantially increases the production cost of the process and makes implementation of the commercialized and large-scale application of the process impossible.
However, in the course of the subsequent study, the inventors discovered a series of new technical problems. The combustion-supporting air fed into the rotary kiln contains a certain amount of water (the water fed into the rotary kiln because of air humidity), and the water reacts with P2O5 in air-flow in the rotary kiln to form metaphosphoric acid (HPO3), and then metaphosphoric acid generated reacts with the dust in the kiln gas to form complicated metaphosphate, at the kiln tail of the rotary kiln, and gradually forms a kiln tail ring in the cylinder at the kiln tail of the rotary kiln (namely the ring formed in the kiln tail of the rotary kiln) and grows up constantly; after a period of working of the kiln, the grown up ring formed in the kiln tail seriously hinders the movement of pellets and makes the pellets fed into the rotary kiln to be returned out of the kiln from the kiln tail, and then the normal working of the process is disrupted and the working of the kiln must be stopped for cleaning of the kiln, and the result further reduces the operation efficiency of the rotary kiln and increases the production cost of phosphoric acid and makes it difficult for the commercial applications of the process.
Therefore, to improve the production efficiency of the current kiln phosphoric acid process, reduce the production cost and ensure the stable operation of the process, the current kiln phosphoric acid process need to be modified and improved in those skilled in the art.